A method for checking the operability of a tank-venting system is disclosed, for example, in U.S. Pat. No. 6,131,550. A tank-venting system for a motor vehicle is, for example, disclosed in international patent application PCT/DE 01/01141, filed Mar. 23, 2001.
The maintenance of specific limit values with respect to the emission performance of vehicles having internal combustion engines is required because of national and international regulations. These regulations pertain especially to the emission of hydrocarbons. In this context, the portion of the hydrocarbons, which is emitted via a leakage of the tank-venting valve, must be taken into account and possibly be eliminated.
In overpressure methods, overpressure is introduced by means of a pressure source into the tank system, which is closed tightly with respect to the ambient. This overpressure must again be reduced after ending the diagnosis. In most cases, the overpressure is reduced to the ambient through the active charcoal filter and this takes place in that the active charcoal filter check valve is opened. With a high charge of the active charcoal filter with hydrocarbons, the danger is present that hydrocarbons can reach the ambient because of this overpressure reduction. To prevent this, it could be provided that the tightness check can be carried out only when there is a hydrocarbon charge of the active charcoal filter which is previously recognized as low. However, this would mean considerable limitations.